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Fasting - The Fact File!

Fasting has become an ever increasingly popular dieting trend used for predominantly weight loss but also for general health, so let’s get the facts straight!

Fasting in of itself is simply a prolonged period in which we do not consume any food and or calorie-containing liquid. There are a variety of fasting protocols ranging from intermittent fasting/time restricted feeding (that is, fasting for most of the day and having a small window of time in which you can eat) to whole day (or longer!) fasts.

Fasting has several associated benefits. As it relates to dieting, the obvious being that it may result in a calorie restricted state. If we reduce the time and/or days that we can eat, we are more likely to eat a quantity of food that would put us in a calorie deficit and therefore initiate weight loss. Of course, calories are simply the energy we get from food. If we consume less calories than the amount we require to maintain our weight, we are considered to be in a calorie deficit which will facilitate weight loss.

What are the other major benefits of fasting?

  • Reduced inflammation

  • Reduced calorie

  • Reduce blood pressure

  • Increased insulin sensitivity and has been associated with improving, if not reversing, the metabolic syndrome (a cluster of symptoms which can lead to heart disease) in animal models. The challenge is that in human, the group we typically see metabolic syndrome in (those who are overweight/obese) may find protocols like fasting difficult to maintain

  • Time restricted feeding has been shown to have benefits in individuals suffering from IBS

  • Increased anabolism (building stuff up). When we fast for periods of time our body becomes more sensitive to the next source of nutrients it takes in (similar to exercise). We can exploit this by targeting a high protein intake in addition to a serving of carbohydrate if the goal is to improve lean body mass and or recover from exercise

  • Improved cognitive function

  • Fasting, which results in a low glycogen state/depleted state, may enhance our adaptation to aerobic training stress (also known as mitochondrial biogenesis)

  • Fasting may induce less neuronal dysfunction and degeneration. Studies have shown fewer clinical symptoms in animal models of Alzheimer’s disease, Parkinson’s disease and Huntington’s disease with fasting

Are there any negatives to fasting?

  • Increased hunger (we produce less leptin when fasting, the hormone responsible for telling us we are full), means fasting can be quite mentally challenging

  • Reduced blood sugar can lead to fatigue, irritability and reduced exercise performance

  • Reduced sleep quality

  • Metabolic adaptation - A reduction in total daily energy expenditure if the fasting is too prolonged, too frequent or chronic (which it shouldn’t be)

  • May result in binge eating or choosing the wrong foods when the fast is broken

  • May lead to micronutrient insufficiencies

How often should you fast?

It depends... It depends on the method of fasting you use, as well as the demands of your current lifestyle and health state. The more experienced an individual becomes the more days/time you could, in theory, fast for.

How to Fast Safely

Much like exercise, you don’t jump in at the hardest point and hope to be successful straight off the bat. We ease ourselves in and steadily progress overtime as our body adapts. This is especially true for those attempting 24 hour fasting protocols. We advise planning your (nutrient rich) meals for when you break the fast to avoid choosing the wrong foods or over-eating/binging.

Be mindful to consume enough water and other liquids (zero kcal variety) throughout your fast to remain hydrated, as you won’t be getting the usual hydrate from food. Fasting can also cause constipation, so ensure you are consuming enough fibre on your non-fasting days to mitigate this risk.

Fasting can be a useful tool but remember, it is our energy balance that will always be the defining factor in fat loss.

Fasting is not advised for pregnant women, individuals with a history of eating disorders and/or those who are underweight. As always, speak to your GP/healthcare professional before you attempt any form of dietary protocol.

The Literature

Last year Harris et al., (2018), produced a systematic review and meta-analysis on 24 hour fasting periods. The study included data from alternate day fasting, fasting for two days, and up to four days per week research.

This meta-analyses showed that this intermittent energy restriction was more effective than no treatment for weight loss and equally as effective in comparison to continuous energy restriction. This is an important point to consider. Fasting isn’t a miracle tool, it’s simply just another tool in the toolbox of weight loss techniques we can use. It won’t work for everyone but it may work for some and, if performed safely and intelligently, there’s no harm in trying!

When we look at more clinical markers of health, a review by Tinsley & La Bounty (2015) found that “Alternate-day fasting trials of 3 to 12 weeks in duration appear to be effective at reducing body fat (≈3-5.5 kg), total cholesterol (≈10%-21%), and triglycerides (≈14%-42%) in normal-weight, overweight, and obese individuals. Whole-day fasting trials lasting 12 to 24 weeks also reduce body fat, and favourably improve blood lipids (≈5%-20% reduction in total cholesterol and ≈17%-50% reduction in triglycerides).”These are important findings, as higher body fat, blood triglycerides and cholesterol are associated with increased risk of heart disease and metabolic syndrome.

Will I Get Hungry While Fasting?

This is highly interdependent (i.e. it depends on the person) and depends on personal perception of ‘hunger’, but leptin will decrease during a fast. However, there is evidence to suggest that if we consume liquid (especially in large volume and or carbonated alternatives) which ‘fills’our stomach, it can trigger the release of leptin and could be an effective way to counteract the hunger. Additionally, caffeine is also effective at reducing perceived ratings of hunger and could also be a useful tool for dieters to use.

Does Fasting Cause Muscle Loss?

It depends on the length of the fast, the type of training you’re doing (if any), whether you are consuming protein to preserve lean tissue and the overall severity of the resulting calorie deficit.


Fasting is a relatively safe, effective tool for weight/fat loss for the average individual/ gym goer/athlete. It should be used in conjunction with energy balance protocols to provide the best outcomes!


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